Very ampleness for Theta on the compactified Jacobian

Abstract

The Jacobian $J$ of a complete, smooth, connected curve $X$ admits a canonical divisor $\Theta$, called the Theta divisor. It is well-known that $\Theta$ is ample and, in fact, $3\Theta$ is very ample. For a general complete, integral curve $X$, D'Souza constructed a compactification $\bar J$ of the Jacobian $J$ by considering torsion-free, rank 1 sheaves on $X$. Soucaris and the author considered independently the analogous Theta divisor $\Theta$ on $\bar J$, and showed that $\Theta$ is ample. In this article, we show that $n\Theta$ is very ample for $n$ greater or equal to a specified lower bound. If $X$ has at most ordinary nodes or cusps as singularities, then our lower bound is 3. Our main tool is to use theta sections associated to vector bundles on $X$ to embed $\bar J$ into a projective space.